A refrigeration system may need multiple compressors in some situation. For example, a parallel connection technology for compressors is used more and more widely in the air conditioning and refrigeration industry. Compressors connected in parallel have advantages, such as convenience in energy adjustment, convenience in maintenance when a single compressor is down, and low costs. Normally, lubricating oil is indispensable while a compressor is running. However, the compressors may have different capacities and different pipe designs, and as a result, a certain compressor, especially, a scroll compressor with a low-pressure chamber may be damaged due to lack of lubricating oil. Therefore, oil levels of the compressors need be controlled. In current oil level control, an active oil-return apparatus widely used in the refrigeration industry may be used, but it has high costs and a complicated system structure, is not applicable to commercial and light commercial air conditioning. It is also possible to adjust the pipe designs to implement oil level control, but this manner cannot reliably control the oil levels of the compressors. Therefore, the current oil level control has higher costs and lower reliability.